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Then Ms Bishop watched the case against the accused Gene Gibson crumble - amid a botched investigation which relied on police interviews which were at best "unreliable", and contained a confession which a Supreme Court judge described as a "complete farce".

The murder charge was downgraded to manslaughter. Gibson pleaded guilty and was sentenced to seven-and-a-half years.

But now Ms Bishop is convinced police had the wrong man all along and 60 Minutes last night followed the mother on her fight to free Gibson.

"Gene Gibson is innocent, this is a no-brainer," she said.

Gibson has been in jail for four years and was charged two years after Josh's murder.

He confessed to the crime and said he killed Josh with a round, metal pole - but Ms Bishop said the story didn't match forensic evidence.

Bartlett reported Sibosado told police he was drunk when he said he killed Josh and police let him go.

"I'm furious," Josh's mother said.

"Senior investigating officer tells me 'we'll get your man'.

"But it was going to be easier to set up Gene Gibson and tick that box rather than do some real investigation work."

Gibson, who since being jailed has protested his innocence, is preparing to appeal his conviction.

In the fallout from the botched investigation, two officers faced disciplinary charges and were fined after pleading guilty, while nine other officers received managerial interventions.

Bartlett says the case is a tragic injustice doled out to a man who could not even understand the charges he was faced with.

"At the crux of it is Gene, a Western Desert Aboriginal, who was let down at every turn," says Bartlett.

"He was 20 years old. Pintubi is his native dialect and his strongest dialect, and then he speaks two other Aboriginal dialects. And in the Supreme Court they actually worked out that English was his fourth and last language.

"But he couldn't speak or understand it very well at all. So he's already cowed by figures of authority, especially police, and he's arrested and gives a so-called confession which is a complete farce. A Supreme Court judge has said that - they talked to him for hours. They didn't have an interpreter. They didn't record the interview ... it's illegal and as depressing as it is wrong."

60 Minutes contacted the West Australian police commissioner to explain the investigation into Josh's death, but declined to comment.